The role of the relieving devices is to allow, under the correct conditions, fluid flow out of or into the process equipment to relieve the build-up of excessive internal or external pressure. The following sections introduce the most common types of relief devices. A more detailed discussion of these devices, including their intended applications, and the advantages and disadvantages of each, will be given in Section 1230. Pressure relief devices are most broadly categorized according to whether or not they are designed to reclose after opening.
1. Nonreclosing Relief Devices
These relief devices are designed to undergo an irreversible mechanical failure at a specified pressure. Their failure allows fluid to flow out of the equipment needing pressure protection, relieving the build-up of excessive pressure. These devices include rupture disks and breaking pin devices.
Rupture Disk Devices
A rupture disk is essentially a membrane that is engineered to contain pressures up to some critical value. When exposed to pressures above this value, the disk fails, allowing fluid to flow through the area formerly blocked by the disk.
Breaking Pin Devices
A breaking pin device contains a load-bearing pin that supports a pressure containing member. The pin is designed to break when the force exerted on it by the pressure-containing member exceeds some critical value. When the pin breaks, the pressure-containing member moves, allowing fluid to flow through the area formerly blocked by the member.
2. Reclosing Relief Devices
This category includes pressure relief valves of several different designs and descriptions. These include spring-loaded, weight-loaded, and pilot-operated pressure relief valves designed to reclose after opening.
Spring-Loaded Pressure Relief Valves
In this design, the valve is held closed against the system pressure by a spring having a specified force. When the system pressure exceeds the spring force, the valve opens, and flow out of the process occurs.
Pilot-Operated Pressure Relief Valves
This design employs two separate valves. The “main” valve, through which relief flow occurs, contains a piston with a larger surface area at its top than at its bottom. Under normal operating conditions, the system pressure is in contact with both sides of this piston. The difference in top and bottom surface areas produces a net force downward, holding the piston closed against the main valve inlet nozzle. The upper side of the piston is also in contact with the inlet of a small, spring-loaded “pilot” valve. When the system pressure exceeds the force of the pilot valve spring, the pilot valve opens, venting the system pressure from the upper side of the main valve piston. The system pressure on the inlet side of the piston then forces the piston to lift, allowing flow through the main valve. When the system pressure drops, the pilot valve recloses, returning system pressure to the top of the piston, and closing the main valve.